Embodiments of the invention relate generally to MR imaging and, more particularly, to a system and method for enhanced contrast MR imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
One area of MR imaging that assists in the creation of high-contrast images is magnetic resonance angiography (MRA). Flow difference peripheral MRA technique is based on physiological flow difference between systole and diastole. This technique relies on the arterial flow difference between the cardiac diastole and systole and acquires images at each of these two cardiac phases. In general, faster arterial flow during systole results in nearly void arterial signal on the systole images while slower arterial flow during diastole results in brighter arterial signal. Since the venous and background signals are relatively independent of the cardiac cycle, a subtraction of the systole images from the diastole images results in arterial only images with good background and venous suppression
However, although the subtracted images depict the systolic faster flowing arteries, arteries that do not exhibit enough of a flow difference between the systole and diastole phases are likely to be subtracted out in a final flow difference image. Thus, for MRA, the potential diagnostic usage of this technique has hitherto been beneficial to arteries which flow slower during diastole and which flow faster during systole.
It has been demonstrated that changing the readout gradient crushers between the two acquisitions can enhance the visualization of the hither-to-slow flowing arteries during systole. This technique, which may be referred as the readout spoiler technique, increases the readout spoiler gradient during systolic acquisition such that even if the flow is slower, such arteries will be dephased, resulting in darker systolic arterial signal. However, for the diastolic acquisition, lesser readout spoiler (or flow compensation) will be used to retain the arterial signal. Although such a schema of modifying the readout spoilers between the two acquisitions benefits in the visualization of slower flowing arteries, it may also result in 1) an increased blurring due to longer echo spacing (i.e., since the spoiler gradient amplitudes need to modified in real-time in between the acquisitions of each of the images), 2) an enhanced background signal on the subtracted image since the eddy currents during the two acquisitions will be different, and 3) the presence of fine line artifacts due to uncrushed free induction decay signal between the refocus pulses since the readout crushers are different between the two acquisitions.
It would therefore be desirable to have a system and method capable of enhancing the visualization of arteries exhibiting a reduced flow difference between the systole and diastole phases while reducing artifacts such as blurring and fine lines. It would also be desirable to enhance the visualization of any tissue that experiences faster signal decay than another tissue.